Third generation (3G) Universal Mobile Telephone communications Systems (UMTS), based on Wideband Code Divisional Multiple Access (WCDMA) radio access, provide wireless access at high data rates and support enhanced bearer services not realistically attainable with first and second generation mobile communication systems. A WCDMA radio access network, like the UMTS Terrestrial Radio Access Network (UTRAN), also enhances quality of service by providing robust operation in fading environments and transparent (soft/softer) handover between base station/base station sectors. For example, deleterious multipath fading is used to improve received signal quality with RAKE receivers and improved signal processing techniques.
Demand continues for improved multimedia communications in the UTRAN including higher peak data rates, lower radio interface delay, and greater throughput. A High Speed-Downlink Shared Channel (HS-DSCH) is standardized for use in WCDMA UTRAN networks to support higher peak rates on the order of 8-14 megabits per second. One of the ways the HS-DSCH achieves higher data speeds is by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller, including one or more of the following briefly described below: shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
Shared Channel Transmission and Higher Order Modulation:
In shared channel transmission, radio resources, like spreading code space and transmission power in the case of CDMA-based transmission, are shared between users using time multiplexing. A high speed-downlink shared channel is one example of shared channel transmission. One significant benefit of shared channel transmission is more efficient utilization of available code resources as compared to dedicated channels. Higher data rates may also be attained using higher order modulation, which is more bandwidth efficient than lower order modulation, when channel conditions are favorable.
Link Adaptation and Rate Control:
Radio channel conditions experienced on different communication links typically vary significantly, both in time and between different positions in the cell. In traditional CDMA systems, power control compensates for differences in variations in instantaneous radio channel conditions. With this type of power control, a larger part of the total available cell power may be allocated to communication links with bad channel conditions to ensure quality of service to all communication links. But radio resources are more efficiently utilized when allocated to communication links with good channel conditions. For services that do not require a specific data rate, such as many best effort services, rate control or adjustment can be used to ensure there is sufficient energy received per information bit for all communication links as an alternative to power control. By adjusting the channel coding rate and/or adjusting the modulation scheme, the data rate can be adjusted to compensate for variations and differences in instantaneous channel conditions.
Channel Dependent Scheduling and Hybrid ARQ:
For maximum cell throughput, radio resources may be scheduled to the communication link having the best instantaneous channel condition. Rapid channel dependent scheduling performed at the bases station allows for very high data rates at each scheduling instance and thus maximizes overall system throughput. Hybrid ARQ with soft combining increases the effective received signal-to-interference ratio for each transmission and thus increases the probability for correct decoding of retransmissions compared to conventional ARQ. Greater efficiency in ARQ increases the effective throughput over a shared channel.
FIG. 1 illustrates a high speed shared channel concept where multiple users 1, 2, and 3 provide data to a high speed channel (HSC) controller that functions as a high speed scheduler by multiplexing user information for transmission over the entire HS-DSCH bandwidth in time-multiplexed intervals. For example, during the first time interval shown in FIG. 1, user 3 transmits over the HS-DSCH and may use all of the bandwidth allotted to the HS-DSCH. During the next time interval, user 1 transmits over the HS-DSCH, the next time interval user 2 transmits, the next time interval user 1 transmits, etc.
High-speed data transmission is achieved by allocating a significant number of spreading codes (i.e., radio resources in CDMA systems) to the HS-DSCH. FIG. 2 illustrates an example code tree with a fixed Spreading Factor (SF) of sixteen. A subset those sixteen codes, e.g., twelve, is allocated to the high-speed shared channel. The remaining spreading codes, e.g., four are shown in the figure, are used for other radio channels like dedicated, common, and broadcast channels.
Although not necessarily preferred, it is also possible to use code multiplexing along with time multiplexing. Code multiplexing may be useful, for example, in low volume transmission situations. FIG. 3 illustrates allocating multiple spreading codes to users 1, 2, and 3 in code and time multiplexed fashion. During transmission time interval (TTI) 1, user 1 employs twelve codes. During transmission time interval 2, user 2 employs twelve spreading codes. However, in transmission time interval 3, user 1 uses two of the codes, and user 3 uses the remaining ten codes. The same code distribution occurs in TTI=4. In TTI=5, user 3 uses two of the codes while user 2 uses the remaining codes.
To achieve higher throughput and high peak data rates, a high speed shared channel may not use closed loop power control, (as dedicated channels do), but instead simply uses the remaining power in the base station cell up to a preset maximum. Because the high speed shared channel is used along with other channels, radio resources must be allocated to the different channels efficiently and without overloading the cell with too high of a power level. The power level for channels other than the high-speed shared channel must be managed to leave sufficient power for the shared channel to have the desired, high throughput.
The code assignment affects the throughput on the high-speed shared channel as well as the available code space for other channels. An optimal code assignment depends on several factors, such as traffic load, the type of traffic, and current radio conditions. If too many CDMA codes are assigned to the high-speed shared channel, some of those codes may be underutilized, which is a waste of radio resources. If too few codes are assigned, the channel throughput over the high-speed shared channel is too low.
The radio network controller (RNC) performs radio resource management. Radio resources like spreading codes are allocated using one or more resource management algorithms. Other examples of such resource management include power/interference control, admission control, congestion control, etc. The radio network controller can better perform its resource management tasks if it knows the current resource status or use in the cell. One measurement useful to the radio network controller is how often the codes currently allocated to the high-speed shared channel are being used. The technology described herein provides measurements from the base station to the radio network controller about the usage of the set of codes currently allocated to a particular channel, like a high speed shared channel. Based on those measurements, the RNC can adjust (if necessary) the code allocation to the high speed shared channel.
Another managed radio resource that needs judicious allocation to different radio channels in a base station cell is radio transmission power level. FIG. 4 shows a graph of base station cell power against time. The radio transmission power for one or more common channels, shown in the bottom graph, takes up a first portion of the allowed or maximum cell power. On top of the common channel power is the combined radio transmission power currently allocated to the dedicated channels. The hatched portion shows the radio transmission power that can be used by the high-speed shared channel. At time tm, the combined common and dedicated channel power equals the maximum cell power. As a result, the high speed shared channel has no available power, and therefore no throughput, assuming the maximum cell power level is observed. On the other hand, if the high speed shared channel uses more than the maximum cell power, signals may be distorted leading to degraded quality of service.
On request from the RNC, base stations may provide measurements to the RNC, e.g., channel quality estimates for rate selection. But such base station measurements do not take into account the special nature of a high-speed downlink shared channel (HS-DSCH). Indeed, one typical base station measurement provided to the RNC is total transmitted carrier power for all downlink channels. That measurement would include the transmission power for the high-speed shared channel. Including the high-speed downlink shared channel in the total transmitted carrier power measurement presents a problem. First, the HS-DSCH, by design, typically uses all of the remaining transmission power up to the cell maximum. Second, the RNC uses the total transmission power measurement to decide whether to set up new dedicated radio channels. Consequently, the RNC will always conclude that the cell is operating at full capacity as long as there is a moderate traffic demand on the high-speed downlink shared channel. For the same reason, channel requests will be denied as soon as there is even moderate traffic demand on the high-speed downlink shared channel. Nor is it possible in this situation to determine an accurate congestion level in the cell. Because the high speed shared channel uses the remaining cell power, the total carrier power measurement will always be equal or close to the cell maximum erroneously suggesting that the cell is always fully loaded.
The technology provides a cell transmission power measurement to a radio resource manager that specifically takes into account a high-speed shared channel even where that channel is designed to use the remaining transmission power in a cell up to a cell maximum. The radio network controller is informed when a high speed shared channel has little or no power available because of increasing power demands required by channels other than the high speed shared channel. Other parameters may also be measured at the base station that may be useful to a radio resource controller.
One or more base station measurements provided to a radio resource manager allows it to optimally access, allocate, and/or regulate radio resources, like spreading codes and transmission power, to different types of radio channels supported in the cell, including a specialized channel like a high-speed shared channel. Such measurements include one or more of the following: other-channel power, HS-DSCH code usage, transport format usage, average active load, empty buffer, excess power, and/or similar parameters.
In one example embodiment, transmission power is measured for signals transmitted over first radio channels that do not include measurement of a transmission power for signals transmitted over a second radio channel, e.g., a high speed shared channel. CDMA code usage may also be measured for the second channel during a predetermined time period. One or both of the measured transmission power and the measured CDMA code usage are reported to a radio resource controller which may take appropriate resource management action(s). In a preferred example, the first and second channels are downlink radio channels from the CDMA mobile communications network to one or more of the mobile radios. The first radio channels include one or more of the following: one or more dedicated channels, one or more common channels, one or more control channels, and one or more broadcast channels. The second channel is a high speed downlink shared channel.
The measured transmission power may be used to perform radio resource control such as power allocation to the second radio channel and/or the first radio channels, code allocation to the second radio channel and/or first radio channel, congestion control, and admission control. The measurement also alerts the radio resource controller to situations where the power being used by the other channels leaves insufficient or rapidly decreasing power for the HS-DSCH. The radio resource controller may take appropriate action to reallocate power resources to ensure there is sufficient power for the HS-DSCH to function.
Using the measured CDMA code usage information, a determination may be made whether CDMA codes currently allocated to the second radio channel are being efficiently used. If not, the current CDMA code allocation for the second radio channel is changed. In one implementation, the predetermined time period includes plural transmission time intervals (TTIs). The number of TTIs that a CDMA code is used for the second radio channel during the determined time period is measured. Alternatively, a number of TTIs that a set of the CDMA codes is used for the second radio channel during the predetermined time period may be measured. The CDMA code usage measurement may be reported in any number of fashions. In one example, the code usage is reported to the resource manager as a histogram.
Other example base station measurements may be used alone or in combination with each other and/or those measurements described above. For example, a number of mobile radio users may be measured that currently have data to transmit over the high speed shared channel in a base station buffer at a data transmission scheduling time for the high speed shared channel. The measured number corresponds to an active load and is provided to a radio network controller for use in managing a load on the high speed shared channel. A buffer monitor may be used to measure an amount of data being buffered per high speed shared channel user. A number of high speed shared channel transmission time intervals (TTIs) is determined over a measurement period when the measured amount of buffered data reaches zero or is below a threshold. The measured number can be used to (re)configure the high speed shared channel. An excess power monitor may be used to measure a first power level actually used for transmission to a mobile radio user over the high speed shared channel and determine a second power level required for reliable transmission to the mobile radio user over the high speed shared channel. The difference between the first and second power levels is calculated and used in allocating resources associated with the high speed shared channel.
The technology enables efficient radio resource management without excessive signaling. By accounting for the specific characteristics of a particular type of channel, like a high-speed shared channel, one or more measurements in accordance with the present invention permits an accurate estimate of current cell conditions. As a result, a radio resource manager can better control cell congestion, admit new users to the cell, block new users, or even drop existing users, if necessary. Actions can be taken to ensure that maximum power limitations are not exceeded before the maximum power is reached which would otherwise result in unpredictable signaling distortion and poor signal quality. Moreover, the technology allows the radio resource controller to ensure the high-speed shared channel has enough resources to fulfill its job as a high-speed shared channel. Since spreading codes are a limited resource in a CDMA system, an optimal code allocation is assured to various channels, which is particularly advantageous for a high-speed shared channel. Proper code allocation to a high-speed shared channel ensures optimal performance of that channel without under-utilizing or otherwise wasting radio resources.